In cryogenic coolers, which utilize the Joule-Thomson effect of cooling a fluid to its liquefraction temperature, a gas under pressure is passed through a nozzle into an expansion chamber. To conserve gas, once the liquefraction temperature has been reached, it is necessary to provide a control valve for regulating the flow of gas through the expansion nozzle. In U.S. Pat. No. 3,517,525, a vapor bulb located in the expansion chamber is connected to a bellows. The bellows holds a needle valve in alignment with the expansion nozzle. As the temperature in the expansion chamber changes, fluid in the vapor bulb correspondingly reacts and allows the bellows to change the position of the needle valve with respect to the expansion nozzle. Unfortunately, the regulation of the needle valve through the operation of the bellows is directly dependent upon the reliability of the vapor bulb. A variety of operational and manufacturing conditions can cause microscopic cracks in the vapor bulb allowing some of the fluid to escape. Although leakage through such microscopic cracks may be of a magnitude too minute to detect by normal production means, failure can occur within one year of shelf-life and less while in use. Once the fluid has escaped from the vapor bulb, the bellows fails to proportionally control the movement of the needle valve as a function of the change in temperature in the expansion chamber.
Later, as disclosed in U.S. Pat. No. 3,827,252, the vapor bulb was replaced with a means to permit gases of different thermal characteristics to be communicated into a bellows and with the gas in the expansion chamber develop an appropriate expanson and contraction rate sufficient to control the flow of gas into the expansion chamber. Unfortunately, the distance that the bellows move the needle valve can create an alignment problem with the orifice which results in improper regulation.
Additionally, U.S. Pat. No. 3,457,730 discloses a valve regulator for a cooler utilizing the Joule-Thomson principle having a temperature sensing element which responds to the temperature differential between the surrounding atmosphere and the expansion chamber. Unfortunately for successful self-regulation, the valve regulator must rapidly sense and respond to changes in temperature in the expansion chamber. In an attempt to increase the effectiveness of this temperature sensing element, fluid from the expansion chamber was communicated essentially throughout the entire length of the cooler. Unfortunately, the temperature of the surrounding atmosphere can continually change resulting in an unstable control. In addition, with the needle valve mounted in the cantilever support it is possible to develop an internal bending movement which can also add to the instability of the control.